The present invention relates to an explosive material composition and a method for producing the same. The explosive material composition of the present invention can be used for explosives which generate gas and/or heat upon combustion, such as a gun propellant, a propellant, a gas generating agent and a pyrotechnic composition. The explosive material composition of the present invention is not directed to a detonating explosive that causes detonation.
Explosive material compositions generally contain solid reactive ingredients. The smaller the size of the solid reactive ingredient is, the larger the surface area, whereby a higher burn rate of the explosive is achieved. Conventionally, an oxidizing agent, which is one of the solid reactive ingredients, is prepared as fine particles. The method of making fine particles includes a pulverization method and a spray drying method. In the pulverization method, for example, after adding an anticoagulant to an oxidizing agent such as strontium nitrate, the oxidizing agent is pulverized. According to the pulverization method, particles of oxidizing agent having an average particle size of 5 to 80 μm.
International Patent Publication No. WO 98/29361 discloses a method of producing a gas generating agent using the pulverization method. In the method of making fine particles of oxidizing agent using a pulverizing apparatus, obtaining particles of oxidizing agent having an average particle size of less than 5 μm is difficult. Even if the particles of oxidizing agent having an average particle size of less than 5 μm are obtained, the fine particles form an agglomerate (see Japanese Laid-Open Patent Publication No. 8-104588). Since the size of the agglomerate is relatively large and influences the properties of explosives including burn rate, the substance is not preferable as an explosive material composition.
U.S. Pat. No. 3,788,095 discloses a spray drying method in which a solution of ammonium perchlorate is sprayed in a cooled chamber to freeze-dry the small droplets of ammonium perchlorate. Japanese Laid-Open Patent Publication No. 8-104588 discloses a technique in which a solution comprising ammonium nitrate, polyacrylamide and water is sprayed in a cooled gas to coagulate and freeze-dry the droplets of the solution. According to the technique, agglomerates having a diameter of 50 to 200 μm comprised of a large number of ammonium nitrate crystals each having a size of 0.5 to 1 μm can be obtained.
The spray drying method, however, requires a coolant, and in order to make fine droplets by spraying, a solution having a low viscosity is necessary. To lowering the viscosity of the solution, a large amount of solvent is necessary. For example, Japanese Laid-Open Patent Publication No. 8-104588 uses water in an amount about 8 times the amount in weight ratio of ammonium nitrate. As the solvent is removed in a large amount when freeze-drying, the agglomerate of ammonium nitrate has micropores about 50% with respect to the volume of the agglomerate. When the agglomerate of oxidizing agent is porous, the bulk density of explosive is decreased, and the amount of chargeable explosive per unit volume is decreased. In addition, since freeze-drying sublimes frozen water in the agglomerate, the content of water in the agglomerate is relatively low. It is technically difficult to mold such agglomerate and the applicable method for the molding is limited to a press method. An extruding method is not applicable.
When an aqueous solution of a binder, such as a thickener, is added to the agglomerate obtained by a pulverization method and a spray drying method, water-soluble oxidizing agent dissolves in the solvent (water). Therefore, the oxidizing agent recrystallizes upon drying and the particles of the oxidizing agent become large and non-uniform.
Water-in-oil emulsion detonating explosives are known in the field of detonating explosives. In a water-in-oil emulsion detonating explosive, an oxidizing agent is present in the form of droplets having a diameter of about 1 μm. Japanese Laid-Open Patent Publication No. 2000-143380 discloses a technique in which a water-in-oil emulsion comprising an aqueous solution of oxidizing agent and liquid fuel is prepared, a resin balloon and a water-absorbing substance, such as cross-linked sodium polyacrylate, are added thereto, and the mixture is poured into an iron chamber to freeze at −196° C. with liquid nitrogen, thereby destroying the emulsion to crystallize the oxidizing agent.
Japanese Laid-Open Patent Publication No. 2001-26490 discloses a water-containing detonating explosive solid composition having reduced tackiness. In the detonating explosive composition, about 50% to 90% of the oxidizing agent is recrystallized by adding, while applying ultrasonic wave, a thickener, such as polyacrylamide, to a water-in-oil emulsion comprising an oxidizing agent, a fuel and an emulsifying agent, and then standing to cool. The oxidizing agent in the detonating explosive has a particle size of 5 to 50 μm.
However, in the technique described in Japanese Laid-Open Patent Publication No. 2000-143380, the water-absorbing substance absorbs water discharged upon the destruction of the emulsion and prevents crystallization of the oxidizing agent which proceeds in the presence of water. According to this action, a uniform particle size is maintained for a long time. Examples of Japanese Laid-Open Patent Publication No. 2000-143380 suggest that by using a water-absorbing substance having a particle size of 200 to 300 μm or 1.5 mm, an oxidizing agent having an average particle size of 5 to 10 μm was obtained. Even if the particle size of the oxidizing agent is small, the obtained material is not preferable because the size of the particles of the ingredient constituting the explosive material composition is not uniform due to the presence of water-absorbing substance of which the size has become larger than the initial size because of water absorption and swelling. Since this water-absorbing substance is locally present and not forming gel, it does not function as a binder. In addition, since Japanese Laid-Open Patent Publication No. 2000-143380 uses a water-absorbing substance, a drying step for removing water is not suggested.
As in Japanese Laid-Open Patent Publication No. 2000-143380, the thickener used for the detonating explosive composition of Japanese Laid-Open Patent Publication No. 2001-26490 absorbs water separated from the detonating explosive and/or coming from outside by scattering semi-swelled powder of the thickener. The composition contains more than 10% of non-recrystallized portions and the particle size of the oxidizing agent is not uniform, ranging from 5 to 50 μm, and therefore, the composition is not preferable as an explosive material composition.
In addition, the detonating explosive compositions described in Japanese Laid-Open Patent Publication No. 2000-143380 and Japanese Laid-Open Patent Publication No. 2001-26490 are water-in-oil emulsion detonating explosives and contain about 4 to 8 mass % of water. Therefore, the composition has low combustion property and low mechanical strength, and cannot be used as an explosive.